The quenching with a gas rather than with water is economically and ecologically advantageous since it avoids the venting of large amounts of steam into the atmosphere and since the gas, after cooling, can be recirculated. In fact, the sensible heat carried by the gas is available for other purposes in a coking plant, particularly for the preheating of a new batch of coal to be charged into a coking chamber. Conventionally, different gases are used for quenching the coke and for preheating and drying the "green" coal of the new charge, the two gases traveling in separate circuits linked by a heat exchanger.
The use of a separately circulating inert gas as a cooling fluid for the coke, coupled with the need for a heat exchanger, is inconvenient and rather cost-intensive. The thermal efficiency of such a system is low because of losses in both circuits and of the poor heat transfer between gases present on the primary and the secondary side of the heat exchanger. The utilization of a single gas for both purposes, on the other hand, creates problems since the water vapors generated in a coal-drying oven give rise to the formation of water gas which reacts with the hot charge to be quenched, with consequent combustion and loss of a significant amount of coke.
Thus, a one-circuit system described in German laid-open specification No. 24 15 758 includes a two-stage condenser wherein the water vapor entrained by the circulating gas is largely eliminated before that gas enters the lower end of a vertical quenching duct or bunker in which the hot coke descends in order to be cooled; the cooling gas, accordingly, traverses that duct in countercurrent to the coke mass.
Even in this system a heat exchanger is used in which the gas leaving the condenser cools the vapor-saturated drying gas enterng same; while this heat exchange expedites the removal of water in the condenser, it causes a not very desirable preheating of the gas reaching the quenching duct as a cooling fluid.
In order to reduce the reliance on a separately generated and correspondingly expensive inert gas in the quenching of glowing coke, it has further been proposed (German laid-open specification No. 30 00 808) to limit the use of such a gas to the lower part of a quenching duct and to pass a flue gas available from a coking chamber through the upper part of the duct forlowering the coke temperature to a point somewhat above 750.degree. C.; the inert gas exiting from the lower part can then serve for the preheating and drying of coal.
The two gases traverse the respective parts of the quenching duct in countercurrent to the descending coke; conventional wisdom, in fact, dictates such countercurrent operations for both the cooling of the coke and the preheating and drying of the "green" coal. An exception to this rule is suggested in German laid-open specification No. 28 53 299 for the specific purpose of maintaining the temperature of the outflowing, heated cooling gas, utilized to operate a steam generator, in the event of a temporary interruption of the coke supply; by branching off part of the inflowing cooling gas from the main lower inlet of the duct to an ancillary upper inlet under these circumstances, that part of the gas is more highly heated so that the resulting mean temperature at the outlet is about the same as under normal operating conditions. Neither of the two last-mentioned German publications addresses the problem of preventing excessive coke losses through combustion due to the presence of water vapor in a gaseous cooling fluid.